The present invention relates generally to a method for forming a metal layer of a semiconductor device, and more particularly, to a method for forming a metal layer of a semiconductor device, which protects the surface of the metal layer from deterioration.
The metallization process is generally regarded as one of the most important aspects of semiconductor device manufacturing technology, since it significantly affects the yield, performance (e.g., speed of operation), and reliability of the devices, particularly as the technology advances toward ultra large-scale integration (ULSI). Metal step coverage was not a serious problem in lesser density prior art semiconductor devices, because of the inherent features of such devices having larger geometries, e.g., contact holes having low aspect ratios (the ratio of depth to width) and shallow steps. However, with the increased integration density of semiconductor devices, contact holes have become significantly smaller, while impurity-doped regions formed in the surface of the semiconductor substrate have become much shallower. Due to the resulting higher aspect ratio of the contact holes and the larger steps, with these current, higher-density semiconductor devices, it has become necessary to improve the conventional aluminum (Al) metallization process in order to achieve the standard design objectives of high-speed performance, high yield, and good reliability of the semiconductor device. More particularly, the utilization of the conventional aluminum metallization process in the fabrication of the higher-density semiconductor devices has resulted in such problems as degraded reliability and failure of the aluminum interconnections due to the high aspect ratio of the contact holes and the resulting poor step coverage of the sputtered aluminum, an increase in contact resistance caused by silicon (Si) precipitation, and degradation of the shallow junction characteristics due to aluminum spiking.
In an effort to overcome these problems of the conventional aluminum metallization process, various new processes have been proposed. For example, for preventing degraded semiconductor reliability caused by the above-mentioned failure of aluminum interconnections, various metallization processes have been proposed, such as those disclosed in Japanese Laid-open Publication No. 62-132348 (by Sugano, et al.,), Japanese Laid-open Publication No. 63-99546 (by Shinpei Ijima), Japanese Laid-open Publication No. 62-109341 (by Masahiro Shimizu, et al.), Japanese Laid-open Publication No. 62-211915 (by Hidekazu Okabayashi, et al.), Japanese Laid-open Publication No. 1-246831 (by Seichi Iwamatsu), Japanese Laid-open Publication No. 59-171374 (by Masaki Satou) and European Patent Application No. 87306084.3 (by Ryoichi Mukai, et al.).
According to the above methods, the contact hole is filled by means of melting and reflowing aluminum or an aluminum alloy. More particularly, in the reflowing step, the aluminum or aluminum-alloy layer is heated beyond its melting temperature, and the thus-melted metal is flowed into the contact hole to fill the same. This reflowing step entails the following drawbacks and disadvantages. First of all, since the semiconductor wafer must be disposed horizontally so as to allow proper filling of the contact hole with the flowing melted material, the throughput of the semiconductor device manufacturing process is lowered. Secondly, the liquid metal layer flowed into the contact hole will seek a lower surface tension, and thus, may, upon solidifying, shrink or warp, and thereby expose the underlying semiconductor material. Further, the heat treatment temperature cannot be precisely controlled, and therefore, desired results are difficult to reproduce. Moreover, although these methods may fill a contact hole with the melted material of the metal layer, the remaining areas of the metal layer (outside the contact hole area) may become rough, thereby impairing subsequent photolithographic process steps. Therefore, a second metallization process may be required to smoothen (or planarize) these rough areas of the metal layer.
As an alternative to melting aluminum or aluminum alloy for filing contact holes, and in order to improve the metal step coverage, a multiple step metallization process is disclosed in U.S. Pat. No. 4,970,176 (by Clarence J. Tracey, et al.). In this process, a predetermined first thickness of a metal layer is deposited on a semiconductor wafer at a cold temperature. Then, the temperature is increased to approximately 400.degree. C. to 500.degree. C., which allows the metal layer to reflow while depositing the remaining and relatively thin second thickness of the metal layer. The reflow of the metal layer takes place through grain growth, recrystallization and bulk diffusion.
According to the Tracy method, the step coverage of a contact hole (via hole) having a high aspect ratio can be improved. However, the aluminum or aluminum alloy cannot completely fill a contact hole having an aspect ratio greater than one and a diameter less than 1 .mu.m, since the aluminum or aluminum alloy is deposited at a high temperature.
In the meantime, Ono et al. have disclosed that when the semiconductor substrate temperature is above 500.degree. C., the Al--Si liquidity suddenly increases (see Proceedings of 1990 VMIC Conference, Jun. 11 and 12, pp76-82). According to this paper, the stress of an Al-1% Si film changes abruptly near 500.degree. C., and the stress relaxation of such a film occurs rapidly at that temperature. Additionally, the temperature of the semiconductor substrate must be maintained between 500.degree. C. and 550.degree. C. in order to fill the contact holes satisfactorily.
Additionally, Yoda Dakashi et al. have suggested a method for manufacturing a semiconductor device which comprises the steps of forming double barrier layers for preventing a reaction between the wiring layer and the semiconductor substrate or an insulating layer, on the inner surface of the contact holes, and then filling the contact holes with a deposited metal such as an Al--Si alloy while heating the semiconductor substrate to a desired temperature of 500.degree. C. to 550.degree. C., as in the Ono et al. paper (refer to Korean Laid-open Patent Publication No. 90-15277 and European Patent Application No. 90104184.0 corresponding to Japanese Patent Application No. 01-061557 filed on Mar. 14, 1989).
According to the Dakashi and Ono methods, an Al--Si film is deposited at a temperature of 500.degree. C. to 550.degree. C. The Al--Si film thus obtained has crystalline particles grown to about ten microns in diameter, which is rather large. Therefore, there is a high probability that the Al--Si film has strong resistance against electron migration, but weak resistance against stress migration. In addition, highly resistant silicon is crystallized at the interfaces between crystalline particles of the Al--Si film. Thus, it is necessary to remove the Al--Si film at the areas other than the contact hole area, and the metallization process becomes unduly complicated. Additionally, since the Al--Si film is deposited at a high temperature, voids are formed in the metal layer, thereby resulting in discontinuities of the metal layer.
Additionally, C. S. Park et al. (which includes one of the present inventors) have disclosed a method for forming a metal wiring layer through a contact hole having a high aspect ratio which comprises the steps of depositing an aluminum alloy to a thickness of 3,000 .ANG. at a temperature below 150.degree. C. and post-heating the deposited aluminum alloy at a temperature of 550.degree. C. for 180 seconds, to thereby completely fill the contact hole with the aluminum alloy (see Proceedings of 1991 VMIC Conference, Jun. 11 and 12, pp 326-328). This method is disclosed in U.S. patent application Serial No. 07/585,218 entitled "A Method For Forming A Metal Layer In A Semiconductor Device," which has been abandoned, and in continuation-in-part application therefor (Ser. No. 07/897,294), now allowed.
FIGS. 1, 2 and 3 illustrate successive steps of a method for forming a metal layer according to the above invention.
FIG. 1 illustrates the formation of a first metal layer. First, a contact hole 22, approximately 0.8 .mu.m in diameter and having a stepped perimeter, is formed in an insulating layer 25 formed on a semiconductor substrate 21. Then, the substrate 21 is put into a sputtering reaction chamber (not shown), and a first metal layer 27 having a thickness of 500-3,000 .ANG. is formed by depositing a metal such as aluminum (Al) or an aluminum alloy, at a temperature of 150.degree. C. or less and under a predetermined degree of vacuum. The first metal layer 27 is comprised of small aluminum grains having a high surface free energy.
FIG. 2 illustrates the filling of the contact hole 22. More particularly, the semiconductor substrate 21 is moved to another reaction chamber (not shown), without breaking the vacuum, and the first metal layer 27 is heat-treated for at least two minutes at a temperature of 550.degree. C., thereby filling the contact hole 22 with the metal. At this time, the pressure in the reaction chamber is preferably as low as possible so that a higher surface free energy is imparted to the aluminum atoms, which can thus more easily migrate into the contact holes. The reference numeral 27a designates a metal layer filling the contact hole.
The heat treatment temperature range in the step depicted in FIG. 2 is between 80% of the melting point (Tm) of the metal and its melting point, and will vary according to the particular aluminum or aluminum alloy employed.
Since the metal layer is heat-treated at a temperature lower than the melting point of aluminum, the metal layer does not melt. For example, the aluminum atoms deposited by sputtering at a temperature below 150.degree. C. migrate upon being heat-treated at 550.degree. C., rather than melting. This migration increases when the surface area is uneven and grainy due to an increase in energy among the surface atoms which are not in full contact with surrounding atoms. Thus, the initially sputtered, grainy layer exhibits an increase in atom migration upon heat-treatment.
FIG. 3 illustrates a step for forming a second metal layer 29. More particularly, the second metal layer 29 is formed by depositing the remainder of the required total metal layer thickness at a temperature selected on the basis of the desired reliability of the semiconductor device, for example, at a temperature below 350.degree. C. This completes the formation of the total (composite) metal layer.
According to the above method, the contact hole can be easily and fully filled with metal, by using the same sputtering equipment used for the conventional deposition method and then annealing the deposited metal. Therefore, even a contact hole with a high aspect ratio can be fully filled by forming and heat-treating a very thin metal layer e.g., about 500 .ANG.. For example, a contact hole with a high aspect ratio (greater than 1.0) and a diameter less than 1 .mu.m can be completely filled. Also, an etching step, as in the above-mentioned Dakashi method, is unnecessary.
However, when a void is formed in the contact hole or when the step coverage of the metal layer is inadequate, the contact hole cannot be filled up while maintaining the semiconductor wafer and deposited metal layer at a desired temperature and vacuum level. Further, although a secondary metal layer is subsequently formed on the semiconductor wafer having a previously deposited primary metal layer, good step coverage of the contact hole cannot be assured, and the reliability of the manufactured semiconductor device is accordingly degraded. According to the above-mentioned method in which metal is deposited at a lower temperature and the metal is heat-treated at a high temperature (below melting point) so that metal grains are reflowed, the grain boundary is severely dug out, thereby creating grooves between the grains of the metal layer. This phenomenon is called grooving. Generally, after the metal layer is formed at a low temperature and the underlying semiconductor substrate cools, succeeding steps of the overall process for manufacturing the semiconductor device are performed. In one of these succeeding steps, an anti-reflective layer is formed on the metal layer. Due to the grooving, slit-shaped cracks or fissures are formed in the anti-reflective layer.
The grooving phenomenon is due to interfacial tension, which increases if cooling is performed too rapidly, i.e., if the temperature is rapidly reduced from a relatively high to a relatively low temperature.
FIGS. 4A and 4B are schematic views of aluminum grains which illustrate the grooving phenomenon. In the drawings, reference numeral 50 is a semiconductor substrate, reference numeral 52 represents a metal layer made of aluminum or an aluminum alloy, reference numeral 54 represents a groove and reference characters Ga, Gb (or Gb') and Gc are metal grains of the metal layer 52.
When the metal layer 52 of aluminum or aluminum alloy is heat-treated repeatedly at high and low temperatures, grain rotation occurs due to strain relaxation, thereby resulting in dislocation slip within and among the metal grains. Specifically, referring to FIGS. 4A and 4B, the metal layer made of aluminum or an aluminum alloy, has a very strong fiber structure having &lt;111&gt; orientation. However, the (111) plane of certain grains (e.g., grain Gb of FIG. 4A) is slightly inclined and not absolutely parallel with the surface of the aluminum or aluminum alloy layer. As shown in FIG. 4A, grain Gb is inclined by an angle .omega.. When the metal layer 52 of FIG. 4A is heat-treated, grain Gb rotates to have &lt;111&gt; orientation, parallel with the surface of the layer. FIG. 4B shows a grain array after the heat treatment of metal layer 52 of FIG. 4A. Grain Gb' is the grain Gb of FIG. 4A after being rotated to a &lt;111&gt; orientation. Such rotation of the metal grains deepens the unevenness of the metal layer surface and the grooving phenomenon. During cooling, plastic deformation due to tensile strain intensifies the grooving phenomenon.
If such a groove is created when the metal layer of aluminum or aluminum alloy is patterned, chemicals penetrate the layer's surface via the groove, and even when the metal layer is capped, e.g., with titanium nitride (TiN), the capping material is not sufficiently deposited in the grooves, which in effect, negates the capping effect.
FIG. 5 is a cutaway perspective view which illustrates the grooving phenomenon. In FIG. 5, reference numeral 42 represents a diffusion blocking layer of a composite layer consisting of a first diffusion layer of titanium (Ti) and a second diffusion layer of TiN. Reference numeral 44 represents a metal layer of an aluminum alloy. Reference numeral 46 is an anti-reflective layer of TiN. Reference characters G1, G2 and G3 represent grains of the metal layer 44. Reference numeral 48 indicates a groove formed between the grains G1, G2 ad G3. Reference numeral 50 represents a slit-shaped crack formed in the anti-reflective layer 46.
FIG. 6 shows an SEM photograph of the surface of a metal layer made of an aluminum alloy, illustrating the case where, when the metal layer is formed according to the method shown in FIGS. 1, 2 and 3, the grooving phenomenon occurs and the surface of the metal becomes coarse. As can be seen, certain portions of the metal layer exhibit severe grooving. In such portions, the TiN step coverage is so poor that, as shown in FIG. 5, the TiN layer 46 splits. For this reason, when a photoresist layer is developed in a succeeding photolithographic process step, chemicals of the developing solution penetrate the surface so that corrosive pitting occurs, or the chemicals permeate the grains such that a residue remains after etching and creates the potential for electrical shorts.
The present inventors discovered that the grooving phenomenon can be prevented by controlling the rate of cooling of the metal layer and/or by controlling the condition of the surface of an underlayer prior to the deposition of the metal layer, thereby enabling deposition of an aluminum or aluminum alloy whose grains form &lt;111&gt; orientation. The present invention is based on these discoveries.